I am in the process of making the same decision myself. I find the image quality to be equivalent in real-world shots, although when I set up my ISO 12233-type chart, the 100-400 fares ever so slightly better at 400mm. My concern is less about weight, and more about the shorter (retracted) length of the 100-400 compared to the combo. I intend to go on a few subsequent outings with the 70-200 II and 2xIII to see how the combo handles for routine use. If it's okay, I plan to sell the 100-400, with the proceeds going toward a 300/2.8 IS II (for times when my 600/4 IS II is too big to bring).
Ok, I've given the 70-200 II + 2xIII a shakedown for routine use, and I'm keeping the 100-400 (for now...if I'm $1K shy of the funds for the 300/2.8 II, it'll go, or if there's an updated 100-400).
The IQ is fine, there are two issues for me. The first is handling - the combo is not very convenient to carry. I normally retract the 100-400, the push-pull means racking it out is very fast, and I do that automatically as I raise the camera. The 70-200 + 2x is front-heavy, and also doesn't balance well on the BR strap, so it bounces around more. I can balance it by sliding the lens plate to the opposite end, but that defeats the anti-twist (not a big deal, it didn't twist anyway), but also means it has to be loosened and moved again to unmount the lens, else the plate hits the body.
The other issue was the AF - the combo is slower than the 100-400, noticeable when it racks out to infinity hunting...and it seemed to hunt more often. The 70-200+2x did fine in good light, but in poorer light with a complex subject (bird in tree branches) or backlit subject, it would hunt more than the 100-400, and sometimes miss focus locking at all, where the 100-400 would have often locked. Those are situations where the 100-400 has some problems, but the 70-200 + 2x was worse.
Agreed! My 100-400 is sitting quite comfortably in my bag right now with no worries other than will she be upgraded for the Ver. II? I go to it 75% more than I do my 70-200 II right now and have ever since grabbing the 5D III last year.
When I had my 7D I seemed to use my 70-200 II moreso than the 100-400. Not so much anymore! I've even thought of letting the 70-200 II go, in order to assist with funding a 300 f/2.8 or the new 500...